Radar systems typically employ antennas that are many times larger than a wavelength of a radar signal transmitted or received by the antenna. An exemplary antenna is a phased array antenna which is composed of numerous radiating elements to provide a narrow beam of radiation, the elements being spaced apart by approximately one-half the nominal wavelength of the radar signal. Signals received by the various radiating elements are combined, as in a monopulse radar, to provide a sum channel signal and a difference channel signal for measurements of elevation and azimuth angles of a radar signal incident upon the antenna. A similar combination of signals takes place in a sonar system employing an array of sonic radiating elements, or transducers, in which a difference channel signal is utilized as in a radar system for indicating the direction of incident radiation relative to the direction at which the transducer array is directed.
The beam produced by the radar antenna is steered by phase shifters coupled to each of the radiating elements. While delay lines may also be used at microwave frequencies, at the present time, the cost and attenuation of such delay lines are excessive so that, generally, only the phase shifters are used. In the case of sonar systems wherein the sonic radiation is of a much lower frequency than the electromagnetic radiation, either phase shifters or delay lines may be utilized for steering the beam, the latter providing a steering capability which is independent of the radiation frequency.
A problem arises in the case of the array antenna utilizing the phase shifters to steer a beam of radiation having a frequency which is variable or wherein several frequency components of the radiated signal are present over a relatively wide bandwidth. A set of phase angles imparted by the phase shifters for steering the beam is present for one value of the radiation frequency with the result that a phase error is present for other values of the radiation frequency. The phase error increases with increasing distance from the center of the antenna, and with increasing frequency offsets from the nominal frequency of the radiation. Noticeable degradation of the radiation pattern of the antenna with attendant diminution in the accuracy of the angle measurements occurs when the bandwidth of the radiated signal is sufficiently wide to introduce phase errors on the order of 30.degree. at the radiating elements near the edge of the array.